1. Field of the Invention
The present invention relates to a process of friction-welding aluminum alloy hollow members, a friction-welded joint, and vehicle parts, such as torque rods, propeller shafts and suspension links or other links, having the joint, in which good tensile strength, fatigue strength and other mechanical properties are ensured.
2. Description of the Related Art
The friction welding process includes the steps of making members abut each other end to end, causing friction between the abutting ends of the members, typically by relative rotation between the abutting ends, to generate friction heat for heating the abutting ends to a welding temperature, and then upsetting the abutting ends by pressing the abutting ends against each other whereby a flash is extruded from the abutting ends.
Friction welding is generally used in many applications because various kinds of materials are bonded without the necessity of special skills as required in MIG or TIG welding. Friction welding also advantageously provides bonding with reduced welding defects and ensures a stable quality. Because of these advantages, friction welding is frequently used for bonding ferrous or iron-based materials.
However, the friction-welded joint unavoidably includes a flash having a root which acts as a stress concentrator to reduce the fatigue strength. Moreover, it is more likely in aluminum-based materials than in ferrous materials that a broader heat affected zone is formed to cause softening of the material because of a higher heat conductivity in comparison with that of ferrous materials. The presence of a flash as a stress concentrator in the heat affected zone, together with the joint material being softer than a base metal, causes cracking, breakage, etc. to occur from the flash root to lower the fatigue strength of the joint. Thus, there remain problems to be solved when friction-welding members of an aluminum-based material and many solutions have been proposed to this end.
For example, Japanese Unexamined Patent Publication (Kokai) No. 5-96385 proposed a process in which the upset timing is set before the members to be friction-welded are brought into contact with each other and an upset pressure is applied to the members for an extremely short time until a rotary motor is stopped. This reduces the size of a heat affected zone, suppresses softening of the base metal near the welded joint and provides a joint having an improved mechanical strength.
However, regulation of the upset timing alone cannot effectively control formation of a flash acting as a stress concentrator. Flashes on the outer surface of a friction-welded product can be relatively easily removed by machining. However, flashes on the inner surface of a friction-welded tubular product cannot actually removed. For example, lightweight vehicle torque rods of aluminum alloys are used to replace those of ferrous materials and are produced by bonding ring form tips at both ends of a tubular cylinder but friction welding is not used because it reduces the fatigue strength. This also applies in vehicle propeller shafts and vehicle links, such as suspension links having a tubular cylinder and tips bonded at both ends of the cylinder.